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The relational and equality operators are left-associative in C. Consequently, C, unlike many other languages, allows chaining of relational and equality operators. Subclause 6.5.8, footnote 107, of the C Standard [ISO/IEC 9899:2011], says:

The expression a<b<c is not interpreted as in ordinary mathematics. As the syntax indicates, it means (a<b)<c; in other words, "if a is less than b, compare 1 to c; otherwise, compare 0 to c."

These operators are left-associative, which means the leftmost comparison is performed first, and the result is compared with the rightmost comparison. This syntax not non-associative as they often are in other languages. A comparison such as{{x<=y<=z}}, for example, is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation. This allows a programmer to write an expression (particularly an expression used as a condition) that can be easily misinterpreted.

Noncompliant Code Example

While Although this noncompliant code example compiles correctly, it is unlikely that it means what the author of the code intended. :

int a = 2;
int b = 2;
int c = 2;
/* ... */
if (a < b < c) /* misleading,Misleading; likely bug */
/* ... */
if (a == b == c) /* misleading,Misleading; likely bug */

The expression a < b < c evaluates to true , rather than false , as its author probably intended, to false, and the expression a == b == c evaluates to false , rather than true , as its author probably intended, to true.

Compliant Solution

Treat relational and equality operators as if it were invalid to chain them.:

if ( (a < b) && (b < c) ) /* clearer,Clearer and probably what was intended */
/* ... */
if ( (a == b) && (a == c) ) /* dittoDitto */

Risk Assessment

Incorrect use of relational and equality operators can lead to incorrect control flow.

Automated Detection

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Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V	chained-comparison	Fully checked
ECLAIR	Include Page ECLAIR_V ECLAIR_V	CC2.EXP13	Fully implemented
GCC	Include Page GCC_V GCC_V		Option -Wparentheses warns if a comparison like x<=y<=z appears

...

; this warning is also enabled by -Wall
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C3392, C3401, C4111, C4112, C4113
LDRA tool suite	Include Page LDRA_V LDRA_V	433 S	Fully implemented
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	503, 731	Fully supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rec.

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Other Languages

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Related Guidelines

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Bibliography

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Risk Assessment

Incorrect use of relational and equality operators can lead to incorrect control flow.

Image Removed      03. Expressions (EXP)      EXP14-C. Beware of integer promotion when performing bitwise operations on chars or shorts